1. Field of the Invention
The present invention generally relates to improving the quality of copies produced by a document imaging device, and more particularly to a method and apparatus for correcting distortions in copies of pages of a book, journal, or other bound volume.
2. Background Description
Conventional digital copy machines produce copies of pages from a bound volume which are often distorted in the area near the binding. This is because the binding prevents the pages from lying flat against the document glass. More specifically, when a book is opened and a page is placed on the glass, the area of the page near the binding curves away (or upwardly) from the glass surface. As a result, printed information (e.g., text, graphics, etc.) near the binding tends to skew when viewed two-dimensionally by the imaging sensors of the machine. If left uncorrected, the skew will transfer as distortion in the output copy.
In an attempt to reduce distortion, users will often press the book against the document glass to flatten the area near the binding. This approach usually proves less than satisfactory especially when the bound volume is thick. Moreover, pressing the volume against the glass has a tendency to damage the book, requiring its replacement sooner than expected.
Various approaches have been developed to reduce distortion in copies of pages taken from a book. One approach, disclosed in U.S. Pat. No. 5,831,750, uses a plurality of charge-coupled device (CCD) sensors to measure the height of the page from various perspectives. The amount of light reflected from the page at each sensor position is used to form a height distribution curve which a processor relies on to correct distortion by adjusting an autofocus function. U.S. Pat. No. 5,808,756 also discloses a which corrects distortion based on page height measurements.
Another approach, disclosed in U.S. Pat. No. 5,835,241, captures two images of a page, a first is taken while a light pattern is projected on the page and a second taken without projection of the light pattern. Using pixels corresponding to the light pattern in the first image, a crease is identified representing the spine of the book. The pixels forming the crease are replaced with ones determined using a parametric model that approximates the spine region. Finally, the second image is de-warped using various transform functions, resulting in the correction of distortion caused by the page curving away from the document platen. U.S. Pat. No. 5,084,611 also discloses an image correction method that uses projected light patterns.
Another approach, disclosed in U.S. Pat. No. 5,497,236, corrects distortion caused by page curvature by fitting lines of text in an unsplayed portion of the page image to a skew line which represents deviation of lines of text in the image from a horizontal reference. Splay is then determined for each line of text, an inverse transformation is performed to straighten the text lines, and horizontal stretching is applied to correct for the projection angle of the original document.
Another approach, disclosed in U.S. Pat. No. 5,818,976, involves examining distortion of the edges of a page. This is accomplished using a background with optical characteristics that contrast with those of the page. Specifically, the page and background are scanned and transitions in the image are detected which define points along one or more of the page edges. A skew angle is then established between the detected edges and a reference orientation, and the image is modified until the resultant skew angle is substantially zero.
The conventional approaches mentioned above require complicated hardware for performing distortion correction. U.S. Pat. Nos. 5,831,750 and 5,808,756 require the use of special equipment for taking height measurements; U.S. Pat. No. 5,835,241 require the projection of light patterns onto the page; and U.S. Pat. No. 5,084,611 uses a two-stage process and an additional light source for detecting the curvature of the page. The approach taken in U.S. Pat. No. 5,497,236 determines splay solely from horizontal lines of text, which could be difficult for non-text documents or documents with many different fonts, diagrams, and nontraditional layouts. The approach disclosed in U.S. Pat. No. 5,818,976 estimates only the edge of a page, which is not representative of the distortion of the page in many cases.
From the foregoing discussion, it is clear that conventional distortion correction approaches have proved less than optimal. A need therefore exists for an improved method of correcting distortion in scanned documents, especially those taken from bound volumes of substantial sizes and thicknesses.